Within the petroleum industry centrifugal pumps are necessary in order to process fluids especially hydrocarbons. Another important application within the petroleum industry is in the mud circuit on a drilling rig. On drilling rigs, mud which consists mainly of water and bentonite as well as of several different additives depending on many different factors is used. The heart of the mud circuit is the mud pump which is in general a high pressure piston pump. It provides the major part of head to overcome the system’s resistance. The mud is pumped through a piping system to the derrick and through the standpipe to a definite high. Now through the kelly hose via the gooseneck into the upper kelly cock. It flows through the Kelly and the lower kelly cock into the drill string down the borehole. At its end, the mud leaves the drilling collars through the drilling bit.
1. Select a pump to handle the highest anticipated flow. Select an impeller size to provide sufficient discharge head to overcome friction in the lines, lift the fluid as required, and have sufficient head remaining to operate the equipment being fed.
Initially this guideline suggested that the pump flange size be selected to provide the highest anticipated flow, even though the flange size has nothing to do with the flow rate. Most pump curves are listed in terms of the flange sizes. The size of the pump impeller housing increases as the flange size increases. An impeller rotating at constant speed will create a constant head independent of the size of the housing or the flanges. An impeller that fits inside a 2×3, 3×4, 4×5, or 5×6 pump will produce the same head in each pump if it is rotated at the same speed. Because the housing of a pump with a 2-inch and 3-inch flange is smaller, the internal friction at a high flow rate will be greater than a 56 pump. This means that the capacity of the various pump sizes will be indicated by their flange sizes. The committee decided to only indicate that the pump should be selected to handle the highest anticipated flow rate, instead of indicating that the flange size is commonly used to specify pump size.
2. Install the centrifugal pump with a flooded suction that is sumped so that sufficient submergence is available to prevent vortexing or air-locking. Foot valves are not needed or recommended with flooded suctions.
A small influx of air into the suction of a centrifugal pump can create cavitation problems and diminished flow. As the air enters the chamber with the impeller, it tends to concentrate in the center of the impeller because of the centripetal acceleration of the drilling fluid.
The liquid continues to move through the pump. The air does not always continue to the impeller tip, but tends to remain in the center of the impeller. This bubble of air forms a barrier for the incoming fluid, which diminishes the flow rate into the pump. The air also experiences a significant decrease in pressure—possibly even below atmospheric pressure. This causes implosions of vapor bubbles that can remove metal from the impeller. The pump will sound as if it is pumping gravel. If it continues in this mode for a long period of time, the impeller will be severely damaged.
Flooded suctions tend to eliminate most of the air influx problems but sometimes a small vortex will form in the mud tank. These small vortexes can entrain a significant amount of area. Increasing agitation in the tanks may prevent a coherent cylinder of air from reaching the suction line. Alternatively, a plate can be installed in the tank to interrupt the formation of a vortex.
In some cases, a centrifugal pump is placed on the ground above a pond or buried tank. Foot valves are needed if the centrifugal pump is operated above the liquid level of the suction tank. Foot valves are check valves that prevent the suction line from draining when the pump is turned off. Care must be taken to eliminate tiny air leaks in the suction line because the absolute pressure will be below atmospheric pressure. The pump and suction line should be filled with fluid before the pump motor is started. Centrifugal pumps do not move air very well.
A centrifugal pump suction can only lift fluid a certain height above a liquid level. These heights are determined by observing the NPSH (negative pressure suction head) values listed on the centrifugal pump curves. If the NPSH is exceeded, cavitation can destroy the impeller.
3. Install a removable screen over the suction to keep out large solids and trash. It can be made out of half-inch expanded metal and should have a total screen area at least five times the cross-sectional area of the suction line so it will not restrict flow. An extended handle arrangement reaching to the tank surface is desirable to allow the screen to be pulled during service and cleaned.
An expanded metal screen prevents objects (such as gloves, buckets, pieces of clothing, chunks of rubber, etc.) from plugging the suction line or fouling the impeller. A bucket, turned so that the bottom fits into the suction line, can be difficult to diagnose and locate. A box made from expanded metal that covers the suction can prevent these disasters. If two alignment yokes are welded to the tank walls to hold a 1-inch pipe handle, the screen can be removed, cleaned, and easily returned to the suction opening. Without these alignment yokes, reseating the expanded metal box is difficult.
4. Suction and discharge lines should be properly sized and as short as practical. Flow velocities should be in the range of 5 to 10 ft/sec. Less than 5 ft/sec causes solids to form a tight layer obstructing the bottom of horizontal lines. At velocities at or exceeding 10 ft/sec, pipe-turns tend to erode, headers do not distribute properly, and usually there will be cavitation in the suction lines. To calculate the velocity inside the pipe, use the following equation:
velocity, ft⁄sec = flow rate, gpm/3.48 (inside diameter, in.)²
Suction lines should contain no elbows, swages, or reducers closer than three (3) pipe diameters to the pump suction flange.
Horizontal pipes will fill with solids until the flow rate reaches 5 ft/sec. Barite in equalizing lines between mud tanks is normally settled until the velocity between the tanks reaches 5 ft/sec. Increasing the diameter of connection lines only causes more barite to settle. Above 10 ft/sec, pressure losses in the pipe become too great. Elbows and swages tend to cause turbulence in the flow stream, which can lead to cavitation.
5. Eliminate manifolding. One suction and one discharge per pump is most cost effective over time. Do not manifold two pumps on the same suction line. Do not pump into the same discharge line with two or more pumps.
Flexibility of piping so fluid can be pumped from any tank through any equipment to any other tank has created more problems over the years than just about any other concept. A properly plumbed system should require only one suction and one discharge for each piece of solids-removal equipment. Ignoring this rule allows rig hands the opportunity to open or close the wrong valves. A leaky or incorrectly opened valve can reduce drilled-solids removal efficiency by up to 50%. This translates to an expensive drilling-fluid system. This problem can be eliminated by storing an extra pump and motor. Arrange the centrifugal pumps and motors so that they may be easily replaced. If a pump or motor fails, simply replace the unit. The damaged unit can be replaced during routine maintenance. Two centrifugal pumps in parallel will not double the head available to equipment because a centrifugal pump is a constant head device. For example, visualize a standpipe that is constantly filled with fluid. If two standpipes of approximately the same height are connected, the flow from both pipes will almost equal the flow from one standpipe. If fluid stands lower in one standpipe than the other. fluid will flow from the highest standpipe to the lowest standpipe. This same flow occurs when two pumps are connected in parallel—fluid will flow backward through one of the pumps.
6. Install a pressure gauge between the pump discharge and the first valve. When the valve is closed briefly, the pressure reading may be used for diagnostic evaluation of the pump performance.
A centrifugal pump uses the smallest amount of power when no fluid is moving through the pump (that is, when the discharge valve is completely closed). If the valve remains closed for longer than 5 minutes, the fluid within the pump will become hot from the impeller agitation. This hot fluid may damage the seals. Closing the valve for a short time allows a good reading of the no-flow head produced by the pump. This reading should be compared with the pump manufacturer’s charts. The diameter of the impeller can then be determined. (A pump may be stamped 5X6X14. This means that it could house a 14-inch impeller but it does not mean that it has a 14-inch impeller. The impeller size is adjusted so the pump will deliver the proper head.) After the pump has been in service for a period of time, the pressure reading will assess the condition of the impeller. This eliminates the need to dismantle the pump for inspection. If the manifold pressure is incorrect, reading the pump no-flow discharge head will assist in troubleshooting.
7. Keep air out of the pump by degassing the mud, having adequate suction line submergence, and installing baffles to break mixer vortices. Properly sized, baffled, and agitated compartments will not vortex unless the drilling fluid level becomes extremely low.
Centrifugal pumps cannot pump aerated fluid. The air tends to gravitate toward the center of the impeller while the liquid moves toward the outside. This creates an air bubble at the center of the impeller. When the air bubble becomes as large as the suction line diameter, fluid will no longer enter the pump. This is called airlock. Only a small cylinder of air vortexing into the pump is sufficient to prevent the pump form moving liquid. Since the air accumulates over a period of time, a small vortex the size of a pencil is sufficient to eventually shut down a 6×8 pump. Baffles are inexpensive and easily installed in an empty tank. Any vertical surface that disrupts the swirling motion of the fluid in a compartment is usually sufficient to destroy a vortex. Rig pump efficiency can decrease from 99 to 85% efficiency if the drilling fluid content rises to 6% volume. Air in the drilling fluid may be calculated by measuring the pressurized and unpressurized mud weight.
8. Do not restrict the flow to the suction side of the pump. Starving the pump suction causes cavitation and this will rapidly damage the pump. When a pump begins cavitating, small vacuum bubbles adjacent to the impeller surface start imploding. The pump sounds as if it is pumping gravel. The implosions quickly remove metal from the surface to the impeller blade. In a very short period of time, holes will appear in the metal. Important: Do not close a valve on the suction line while the pump is running!
Starving the suction will decrease the output head. If the head, or pressure produced by the pump, is too high, change to a smaller diameter impeller. On a temporary basis, a discharge valve can be partially closed. On a long-term basis, however, considerable valve erosion will occur so a new, properly sized impeller is necessary. Even a large centrifugal pump is not damaged if only 10 to 20 gpm is discharged from the pump. In fact, the lower flow rates will require less horsepower to the motor than pumping fluid at a much higher flow rate.
9. Make sure the impeller rotation is correct.
Centrifugal pumps will pump fluid even if running in reverse. The head produced by the pump will be lower than it should be. The pressure gauge installed between the pump and the first valve will assist with the diagnosis. Usually, switching two wires in the lead-in panel box will correct the rotation.
10. Startup procedure for an electric motor–driven centrifugal pump with a valve on the discharge side between the pump and the equipment being operated is to start the pump with the valve just slightly open. Once the pump is up to speed, open the valves slowly to full open. This approach will reduce the startup load on the electric motor and will reduce the shock loading on equipment such as pressure gauges and hydrocyclones.
An alternative startup procedure is to completely close the discharge valve before startup and then open the valve slowly immediately after startup to prevent overheating and possible damage to the pump seals.
An electric motor–driven centrifugal pump will immediately try to produce a constant head when it is turned on. If the pump is pumping into an empty line, the flow rate is enormous. Very high flow rates require very high currents to the electric motor. Circuit breakers can stop the pump and avoid motor burnout. Lower horsepower is required if the pump is started with the discharge valve closed.